The receptor for immunoglobulin E (IgE) mediates reactions of considerable medical interest; in addition it serves as a more general model system for effector systems activated by immunoglobulins. Our basic aim is to understand the early events triggered by this receptor in molecular terms. For this purpose we are determining the structure of the receptor, exploring the early events it activates and ultimately will attempt to understand the structural-functional relationships. During the past year the following new results were obtained: 1. Completion of our compositional analyses (including biosynthetic incorporation studies) have provided rigorous criteria for defining the alpha, beta and gamma subunits of the receptor, 2. A cDNA that codes for the alpha chain has been isolated and sequenced and considerable progress has been made in isolating the cDNA for the beta chain, 3. Monoclonal antibodies to the alpha and beta chains have been prepared and used to examine unresolved questions regarding the receptor, 4. Preparation of cytoplasts has been successful and these retain substantial receptor-mediated hydrolysis of phosphoinositides (PI), 5. Further study of PI breakdown by intact cells has shown that in addition to the previously recognized Ca-dependent PI breakdown, the tumor cells with which we work also have a Ca- independent PI breakdown. This occurs very early and leads to the formation of the active 1,4,5 inositol trisphosphate. The latter produces a subsequent mobilization of internal stores of Ca, 6. Evidence has been developed that upon activation of the receptors, protein kinase C is recruited to the membrane.